The subject matter described herein relates generally to medical imaging systems, and more particularly, to a multiplexing device utilized in a medical imaging system.
In PET imaging, the general locations of annihilation photons impacting a detector are recorded. The outputs from the detector are then utilized to reconstruct an image of a patient. More specifically, when a photon impacts a crystal on the detector, the crystal generates a signal. When the signal exceeds a predetermined threshold, an event detection pulse (EDP) is generated. The EDP is then provided to a processor coincidence circuitry which identifies approximately simultaneous EDP pairs which correspond to crystals in the detector which are generally on opposite sides of an imaging area.
The processor coincidence circuitry includes a Time-to-Digital Convertor (TDC) to measure the arrival time of the detected annihilation photons by measuring a time difference between annihilation events using a universal clock. More specifically, the TDCs measure the arrival time of each of the detected photons by measuring a time difference between pulses generated by the universal clock and a leading edge of a logic signal generated by a timing pick off circuit (TPO). Thus, a simultaneous pulse pair indicates that an annihilation event has occurred on a straight line between an associated pair of crystals. Over an acquisition period of a few minutes, millions of annihilation events are recorded, wherein each annihilation event is associated with a unique crystal pair in the detector.
However, high-resolution TDCs are generally large in size and consume relatively large quantities of power to operate effectively. Thus, at least one known detector combines multiple signals output from the detector to reduce a quantity of signals processed by the TDCs thus reducing the power consumed by the TDCs and also reducing the quantity of TDCs utilized to process the signals.
However, for silicon photomultiplier (SiPM) based detectors, the dark pulse rate generated by thermal created electron-hole pairs may affect the timing resolution of the SiPM based detectors. For example, as the dark pulse rate increases, the timing performance is reduced. As a result, the dark pulse rate in a TPO circuit increases linearly with the total area of the SiPM's being processed by each TPO. The detector's timing performance is therefore affected by the area processed by the TPO resulting in a relatively large quantity of TPO circuits being utilized to process the signals.
More specifically, to achieve an optimal timing performance, a TPO circuit is used for each crystal. However, if a TDC is utilized for each crystal, the quantity of TDCs in the detector becomes large thus increasing the power consumed.